The overall goal ofthe research proposed In this application overall is to understand how the innate immune system regulates adaptive immune responses, and to harness this understanding in designing vaccines. The subject ofthe research funded by the present grant during the current cycle focused on the central problem of how/ the immune system launches robust immunity against invading pathogens, while maintaining tolerance to self. This problem assumes a particular significance in the intestine because ofthe trillions of commensal microorganisms and food antigens that confront the intestinal immune system every day. Recent advances suggest that DCs and macrophages play a fundamental roie in maintaining the balance between immunity and tolerance. The hypothesis ofthe application was that balance between immunity and tolerance in the intestine is a complex function ofthe subset of antigen-presenting cell (APC), the microbiota, and instructive signals from stromal elements. This hypothesis is being tested in the following specific aims: Aim 1; To determine whether distinct subsets of lamina propria DCs and macrophages differentially bias the class of innate and adaptive immune responses. Aim 2: To determine whether commensal bacterial flora regulate the functions of lamina propria DCs and macrophages and their ability to induce Thi 7 versus T regulatory responses. Aim 3 : To determine the innate responses of lamina propria DCs and macrophages to oral administration of adjuvants or vaccines, and the effects of such responses on the adaptive immune response. Research performed in each of these Aims have yielded exciting and unexpected results. Forexample, we have characterized the phenotypes, functions and regional localization of intestinal APC subsets, and defined a novel transcription factor that programs intestinal DCs to induce T regulatory responses. Future work is aimed at exploring additional transcriptional networks that program DCs and macrophages to a tolerogenic state. RELEVANCE (See Instructions): Understanding the cellular and molecular mechanisms by which DCs control T cell responses in the inestine and periphery will be of great value in the rational design of therapeutics against inflammatory disorders and vaccines against infections.